How Marijuana Really Affects the Brain


Clearing the air: New science reveals that toking up may be more addictive than previously thought. (Deux/The Image Bank/Getty Images)

Although there’s a lot of buzz about marijuana being nonaddictive, the evidence is stacking up that people can — and do — become dependent on the drug. A study released earlier this year, for example, found that 40 percent of marijuana users in an outpatient treatment program showed signs of withdrawal, a classic indicator of addiction. Now, new research in the journal PNAS sheds light on how lighting up changes the brain — and potentially primes people for withdrawal.

It’s long been known that exposure to THC, the primary psychoactive ingredient in pot, can lead to changes in the brain. Problem is, different studies have shown different structural alterations, making it difficult to pinpoint exactly how the stuff affects people mentally. That’s why a group of researchers decided to use three different magnetic resonance imaging (MRI) techniques to examine the brains of 48 chronic marijuana users and 62 nonusers, while also assessing IQ and negative life consequences of pot smoking.

The most obvious difference: The people who regularly toked up had less volume in the orbitofrontal gyri. This brain region is part of the orbitofrontal cortex, “one of the primary areas within the reward system, which is basically a network of brain regions implicated in the addiction process,” said study author Francesca Filbey, an addiction researcher at the Center for BrainHealth at the University of Texas at Dallas. “More specifically, the orbitofrontal cortex is important for decision-making. This is the area of the brain that would learn something is good for us or bad for us.”

So why does pot cause shrinkage in this area? Simple: The orbitofrontal cortex is highly concentrated with cannabinoid receptors, the places in your brain where THC binds. As a result, it’s much more vulnerable to the effects of a chronic flood of the substance. In animal studies, “the number of those receptors decreased as a result of THC exposure as a way to regain balance in that system,” Filbey told Yahoo Health. “So too much THC basically leads to lower numbers of those receptors in the brain.”

Related: 10 Ways Pot Can Make You Healthier

It’s this effect that gives credence to the “pot is addictive” camp. As Filbey explained, the fewer cannabinoid receptors a marijuana user has, the more THC he requires to achieve the desired high. “This really describes tolerance,” she said. “Around 10 percent of users, on average, report changes in tolerance and also increased craving and withdrawal.” The marijuana users in this study weren’t just casual smokers — they used the drug at least four times per week.

Although this study may help illuminate the addiction process, the findings are a little hazier when it comes to how these brain changes affect people’s behavior and intelligence, if at all. The researchers did find that marijuana users scored lower on an IQ test than non-users did. But as tempting as it is to link this to the changes in their brains, the researchers weren’t able to firmly establish that connection, suggesting there’s another factor behind the users’ lower IQs. One possibility: “If these individuals were using during their adolescent years, then they may have missed a lot of the verbal knowledge that IQ is testing for,” Filbey said.

Related: Pot-Smoking Mom Fights to Breastfeed. What Are the Risks?

And, incredibly, the brain seems capable of compensating for the volume loss associated with consistent marijuana use: The pot smokers showed increased connectivity in the orbitofrontal cortex. What does that mean, exactly? “Structural connectivity refers to the actual white matter tracks that connect the gray areas in our brain,” said Filbey. “Functional connectivity is how well brain regions coactivate — if they respond synchronously. This basically suggests there’s greater communication within the network.” The positive effects on connectivity were greatest in people who’d started using marijuana at a young age.

If this sounds too good to be true, that’s because it is — at least if you’ve been toking up for years. After about six years of chronic pot smoking, these compensatory increases in connectivity began to reverse. “[Connectivity] actually started to decline,” Filbey said. “[The brain] isn’t able to sustain itself past continued use of about six years.”

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